Each of these devices selects four bits of data from two sources under the control of a common Select input (S). The Output Enable (OE) is active LOW. When OE is HIGH, all of the outputs (Y or Y) are in the high-impedance state regardless of all other input conditions.

Moving data from two groups of registers to four common output buses is a common use of the ’AC257, ’ACT257, and CD74ACT258. The state of the Select input determines the particular register from which the data comes. The ’AC257, ’ACT257 and CD74ACT258 can also be used as function generators.